Cartons having bellows folded tops are in widespread use for the packaging of liquids, especially milk, fruit juices and soft drinks. A persistent problem with cartons of this type is the difficulty which is experienced in opening the cartons to provide a pouring spout. Cartons of this type have a top formed from top-forming front, rear and opposed side panels in which the front and rear panels are folded in between the side panels along fold lines running on each of the front and rear panels from the bottom corners of the respective panel to meet on the center line of the panel a little way below the top of the panel. Each of the front and rear panels is thereby divided by fold lines into a central isosceles triangular region being flanked on each side by a triangular wing region. These regions are surmounted by a generally rectangular transversely running narrow sub-panel which is incorporated into a sealing fin on the top of the closed carton.
To open such a carton one is directed to peel apart the wing regions of the front panel of the top and then by inward pressure to pop forward the front panel to form a pouring spout. This operation is only straightforward if the marginal top edge portion of the front panel is not strongly sealed internally to the adjacent side portions of the top. Ideally, the front panel of the top would be sealed to the side panel internally only in small corner regions near the common border of the top panel and the adjacent side panel. However, in practice, such an arrangement is not possible if a reliable liquid tight seal is to be formed. It is generally found necessary to adopt some action to prevent a pin-hole leak occurring caused by the difficulty of making a reliable seal at the point where the apex of the folded in rear panel lies. At this point, there is a step in the fin caused by a transition from four thicknesses of paperboard to two thicknesses. Typically, the problem of making a reliable seal at this point is solved by sealing the apex of the front panel to the apex of the rear panel so that the liquid is retained below the step. If this is not done, liquid may pass up above the top edge of the folded-in front and rear panel and may then track forward beneath the internal top seal of the fin and escape at the front of the package at the point in the sealing fin where there is again a step between four thicknesses of paperboard and two thicknesses of paperboard at the corners where the front panel and the side panels join. No satisfactory solution to this problem has been found which does not involve some degree of sealing of the front panel top edge internally with consequent resistance to opening of the pouring spout of the package.
Attempts have been made to overcome this problem by the use of coatings to reduce the strength or minimize the area of the seal, but these coatings are costly and their function dependent on accurate temperature control when heat sealing which is not always achieved in practice.